Torque transfer coupling with magnetorheological clutch actuator

ABSTRACT

A torque transfer mechanism is provided for controlling the magnitude of a clutch engagement force exerted on a multi-plate clutch assembly that is operably disposed between a first rotary and a second rotary member. The torque transfer mechanism includes an actuator having a first cam fixed for rotation with the first rotary member and a second cam having a rotor which is rotatably disposed within a chamber filled with magnetorheological fluid. An electromagnetic coil is disposed in proximity to the chamber and is selectively energized for varying the viscosity of the magnetorheological fluid so as to induce axial movement of the first cam for engaging the multi-plate clutch assembly.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/357,046 filed Feb. 3, 2003.

FIELD OF THE INVENTION

The present invention relates generally to power transfer systems forcontrolling the distribution of drive torque between the front and reardrivelines of a four-wheel drive vehicle. More particularly, the presentinvention is directed to a power transmission device for use in motorvehicle driveline applications and having a magnetorheological clutchactuator that is operable for controlling actuation of a multi-platefriction clutch assembly.

BACKGROUND OF THE INVENTION

In view of increased demand for four-wheel drive vehicles, a plethora ofpower transfer systems are currently being incorporated into vehiculardriveline applications for transferring drive torque to the wheels. Inmany vehicles, a power transmission device is operably installed betweenthe primary and secondary drivelines. Such power transmission devicesare typically equipped with a torque transfer mechanism for selectivelyand/or automatically transferring drive torque from the primarydriveline to the secondary driveline to establish a four-wheel drivemode of operation. For example, the torque transfer mechanism caninclude a dog-type lock-up clutch that can be selectively engaged forrigidly coupling the secondary driveline to the primary driveline toestablish a “part-time” four-wheel drive mode. In contrast, drive torqueis only delivered to the primary driveline when the lock-up clutch isreleased for establishing a two-wheel drive mode.

A modern trend in four-wheel drive motor vehicles is to equip the powertransmission device with an adaptive transfer clutch in place of thelock-up clutch. The transfer clutch is operable for automaticallydirecting drive torque to the secondary wheels, without any input oraction on the part of the vehicle operator, when traction is lost at theprimary wheels for establishing an “on-demand” four-wheel drive mode.Typically, the transfer clutch includes a multi-plate clutch assemblythat is installed between the primary and secondary drivelines and aclutch actuator for generating a clutch engagement force that is appliedto the clutch plate assembly. The clutch actuator can be apower-operated device that is actuated in response to the magnitude ofan electric control signal sent from an electronic controller unit(ECU). Variable control of the control signal is typically based onchanges in current operating characteristics of the vehicle (i.e.,vehicle speed, interaxle speed difference, acceleration, steering angle,etc.) as detected by various sensors. Thus, such “on-demand” powertransmission devices can utilize adaptive control schemes forautomatically controlling torque distribution during all types ofdriving and road conditions.

Currently, a large number of on-demand transfer cases are equipped withan electrically-controlled clutch actuator that can regulate the amountof drive torque transferred to the secondary output shaft as a functionof the value of the electrical control signal applied thereto. In someapplications, the transfer clutch employs an electromagnetic clutch asthe power-operated clutch actuator. For example, U.S. Pat. No. 5,407,024discloses a electromagnetic coil that is incrementally activated tocontrol movement of a ball-ramp drive assembly for applying a clutchengagement force on the multi-plate clutch assembly. Likewise, JapaneseLaid-open Patent Application No. 62-18117 discloses a transfer clutchequipped with an electromagnetic actuator for directly controllingactuation of the multi-plate clutch pack assembly.

As an alternative, the transfer clutch can employ an electric motor anda drive assembly as the power-operated clutch actuator. For example,U.S. Pat. No. 5,323,871 discloses an on-demand transfer case having atransfer clutch equipped with an electric motor that controls rotationof a sector plate which, in turn, controls pivotal movement of a leverarm that is operable for applying the clutch engagement force to themulti-plate clutch assembly. Moreover, Japanese Laid-open PatentApplication No. 63-66927 discloses a transfer clutch which uses anelectric motor to rotate one cam plate of a ball-ramp operator forengaging the multi-plate clutch assembly. Finally, U.S. Pat. Nos.4,895,236 and 5,423,235 respectively disclose a transfer case equippedwith a transfer clutch having an electric motor driving a reductiongearset for controlling movement of a ball screw operator and aball-ramp operator which, in turn, apply the clutch engagement force tothe clutch pack.

While many on-demand clutch control systems similar to those describedabove are currently used in four-wheel drive vehicles, a need exists toadvance the technology and address recognized system limitations. Forexample, the size, weight and electrical power requirements of theelectromagnetic coil or the electric motors needed to provide thedescribed clutch engagement loads may make such system cost prohibitivein some four-wheel drive vehicle applications. In an effort to addressthese concerns, new technologies are being considered for use inpower-operated clutch actuator applications such as, for example,magnetorheological clutch actuators. Examples of such an arrangement aredescribed in U.S. Pat. Nos. 5,915,513 and 6,412,618 wherein amagnetorheological actuator controls operation of a ball-ramp unit toengage the clutch pack. While such an arrangement may appear to advancethe art, its complexity clearly illustrates the need to continuedevelopment of even further defined advancement.

SUMMARY OF THE INVENTION

Thus, its is an object of the present invention to provide a powertransmission device for use in a motor vehicle having a torque transfermechanism equipped with a magnetorheological clutch actuator that isoperable to control engagement of a multi-plate clutch assembly.

It is a further object of the present invention to provide amagnetorheological thrust cam operator and an electromagnet for use asthe clutch actuator in a torque transfer mechanism.

As a related object, the torque transfer mechanism of the presentinvention is well-suited for use in motor vehicle driveline applicationsto control the transfer of drive torque between a first rotary memberand a second rotary member.

According to a preferred embodiment, the torque transfer mechanismincludes a magnetorheological clutch actuator which is operable forcontrolling the magnitude of clutch engagement force exerted on amulti-plate clutch assembly that is operably disposed between the firstrotary member and a second rotary member. The magnetorheological clutchactuator includes a first thrust cam that is fixed for rotation with thefirst rotary member, a second thrust cam, a chamber filled withmagnetorheological fluid communicating with at least one of the thrustcams, and an electromagnet which surrounds a portion of the chamber. Inoperation, activation of the electromagnet creates a magnetic flux fieldwhich travels through the magnetorheological fluid for proportionallyincreasing its viscosity, thereby creating drag which results in anaxial separation force between the thrust cams. This axial separationforce results in axial movement of the first thrust cam for exerting aclutch engagement force on the clutch pack, thereby transferring drivetorque from the first rotary member to the second rotary member. Upondeactivation of the electromagnet, a return spring releases the clutchpack from engagement and acts to axially move the first thrust cam to aneutral position.

In accordance with one preferred embodiment, the chamber is definedbetween the first thrust cam and the second thrust cam. Further, thefirst and second thrust cams have corresponding first and second camsurfaces that are arranged to normally cause common rotation of thefirst and second thrust cams when the electromagnet is deactivated. Uponactivation of the electromagnet, a reaction force is generated betweenthe first and second cam surfaces for causing axial movement of thefirst thrust cam relative to the clutch pack for engaging the clutchpack.

In accordance with an alternative preferred embodiment, a recess isformed in a housing within which the second thrust cam is rotatablysupported. The sealed chamber is defined between the recess and thesecond thrust cam. The viscosity of the magnetorheological fluid iscontrollably varied to induce a drag force on the second thrust cam forimparting the reaction force between the first and second thrust cams.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the present invention willbecome apparent to those skilled in the art from analysis of thefollowing written description, the appended claims, and accompanyingdrawings in which:

FIG. 1 illustrates the drivetrain of a four-wheel drive vehicle equippedwith a power transmission device incorporating the present invention;

FIG. 2 is a schematic illustration of an on-demand 4WD transfer caseequipped with a torque transfer mechanism having a magnetorheologicalclutch actuator and a multi-plate friction clutch;

FIG. 3 is a partial sectional view of an the transfer case showing thetorque transfer mechanism arranged for selectively transferring drivetorque from the primary output shaft to the secondary output shaft;

FIG. 4 is a partial sectional view of alternative embodiment of a torquetransfer mechanism arranged for use in the transfer case of the presentinvention;

FIG. 5 is a modified version of the torque transfer mechanism shown inFIG. 3;

FIG. 6 is a modified version of the torque transfer mechanism shown inFIG. 4;

FIG. 7 is a schematic illustration of an alternative drivetrain for afour-wheel drive vehicle equipped with a power transmission device ofthe present invention; and

FIGS. 8 through 11 are schematic illustrations of alternativeembodiments of the power transmission devices according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to a torque transfer mechanism thatcan be adaptively controlled for modulating the torque transferred froma first rotary member to a second rotary member. The torque transfermechanism finds particular application in power transmission devices foruse in motor vehicle drivelines such as, for example, an on-demandclutch in a transfer case or in-line torque coupling, a biasing clutchassociated with a differential assembly in a transfer case or a driveaxle assembly, or as a shift clutch in a multi-speed automatictransmission. Thus, while the present invention is hereinafter describedin association with particular arrangements for use in specificdriveline applications, it will be understood that theconstruction/applications shown and described are merely intended toillustrate embodiments of the present invention.

With particular reference to FIG. 1 of the drawings, a drivetrain 10 fora four-wheel drive vehicle is shown. Drivetrain 10 includes a primarydriveline 12, a secondary driveline 14, and a powertrain 16 fordelivering rotary tractive power (i.e., drive torque) to the drivelines.In the particular arrangement shown, primary driveline 12 is the reardriveline while secondary driveline 14 is the front driveline.Powertrain 16 includes an engine 18, a multi-speed transmission 20, anda power transmission device hereinafter referred to as transfer case 22.Rear driveline 12 includes a pair of rear wheels 24 connected atopposite ends of a rear axle assembly 26 having a rear differential 28coupled to one end of a rear prop shaft 30, the opposite end of which iscoupled to a rear output shaft 32 of transfer case 22. Front driveline14 includes a pair of front wheels 34 connected at opposite ends of afront axle assembly 36 having a front differential 38 coupled to one endof a front prop shaft 40, the opposite end of which is coupled to afront output shaft 42 of transfer case 22.

With continued reference to the drawings, drivetrain 10 is shown tofurther include an electronically-controlled power transfer system forpermitting a vehicle operator to select between a two-wheel high-rangedrive mode, a part-time four-wheel high-range drive mode, an on-demandfour-wheel high-range drive mode, a neutral non-driven mode, and apart-time four-wheel low-range drive mode. In this regard, transfer case22 is equipped with a range clutch 44 that is operable for establishingthe high-range and low-range drive connections between an input shaft 46and rear output shaft 32, and a power-operated range actuator 48 that isoperable to actuate range clutch 44. Transfer case 22 also a transferclutch 50 that is operable for transferring drive torque from rearoutput shaft 32 to front output shaft 42 for establishing the part-timeand on-demand four-wheel drive modes. The power transfer system furtherincludes a power-operated mode actuator 52 for actuating transfer clutch50, vehicle sensors 54 for detecting certain dynamic and operationalcharacteristics of the motor vehicle, a mode select mechanism 56 forpermitting the vehicle operator to select one of the available drivemodes, and a controller 58 for controlling actuation of range actuator48 and mode actuator 52 in response to input signals from vehiclesensors 54 and mode selector 56.

Transfer case 22 is shown schematically in FIG. 2 to include a housing60 from which input shaft 46 is rotatably supported by a bearingassembly 62. As is conventional, input shaft 46 is adapted for drivenconnection to the output shaft of transmission 20. Rear output shaft 32is shown rotatably supported between input shaft 46 and housing 60 viabearing assemblies 64 and 66 while front output shaft 42 is rotatablysupported between transfer clutch 50 and housing 60 by a pair oflaterally-spaced bearing assemblies 68 and 69. Range clutch 44 is shownto include a planetary gearset 70 and a synchronized range shiftmechanism 72. Planetary gearset 70 includes a sun gear 74 fixed forrotation with input shaft 46, a ring gear 76 fixed to housing 60, and aset of planet gears 78 rotatably supported on pinion shafts 80 extendingbetween front and rear carrier rings 82 and 84, respectively, that areinterconnected to define a carrier 86.

Planetary gearset 70 functions as a two-speed reduction unit which, inconjunction with a sliding range sleeve 88 of synchronized range shiftmechanism 72, is operable to establish either of a first or second driveconnection between input shaft 46 and rear output shaft 32. To establishthe first drive connection, input shaft 46 is directly coupled to rearoutput shaft 32 for defining a high-range drive mode in which rearoutput shaft 32 is driven at a first (i.e., direct) speed ratio relativeto input shaft 46. Likewise, the second drive connection is establishedby coupling carrier 86 to rear output shaft 32 for defining a low-rangedrive mode in which rear output shaft 32 is driven at a second (i.e.,reduced) speed ratio relative to input shaft 46. A neutral non-drivenmode is established when rear output shaft 32 is disconnected from bothinput shaft 46 and carrier 86.

Synchronized range shift mechanism 72 includes a first clutch plate 90fixed for rotation with input shaft 46, a second clutch plate 92 fixedfor rotation with rear carrier ring 84, a clutch hub 94 rotatablysupported on input shaft 46 between clutch plates 90 and 92, and a driveplate 96 fixed for rotation with rear output shaft 32. Range sleeve 88has a first set of internal spline teeth that are shown meshed withexternal spline teeth on clutch hub 94, and a second set of internalspline teeth that are shown meshed with external spline teeth on driveplate 96. As will be detailed, range sleeve 88 is axially moveablebetween three distinct positions to establish the high-range, low-rangeand neutral modes. Range shift mechanism 72 also includes a firstsynchronizer assembly 98 located between hub 94 and first clutch plate90, and a second synchronizer assembly 100 disposed between hub 94 andsecond clutch plate 92. Synchronizers 98 and 100 work in conjunctionwith range sleeve 88 to permit on-the-move range shifts.

With range sleeve 88 located in its neutral position, as denoted byposition line “N”, its first set of spline teeth are disengaged from theexternal clutch teeth on first clutch plate 90 and from the externalclutch teeth on second clutch plate 92. Thus, no drive torque istransferred from input shaft 46 to rear output shaft 32 when rangesleeve 88 is in its neutral position. When it is desired to establishthe high-range drive mode, range sleeve 88 is slid axially from itsneutral position toward a high-range position, denoted by position line“H”. First synchronizer assembly 98 is operable for causing speedsynchronization between input shaft 46 and rear output shaft 32 inresponse to sliding movement of range sleeve 88 from its neutralposition toward its high-range position. Upon completion of speedsynchronization, the first set of spline teeth on range sleeve 88 moveinto meshed engagement with the external clutch teeth on first clutchplate 90 while its second set of spline teeth are maintained inengagement with the spline teeth on drive plate 96. Thus, movement ofrange sleeve 88 to its high-range position acts to couple rear outputshaft 32 for common rotation with input shaft 46 and establishes thehigh-range drive mode connection therebetween.

Similarly, second synchronizer assembly 100 is operable for causingspeed synchronization between carrier 86 and rear output shaft 32 inresponse to axial sliding movement of range sleeve 88 from its neutralposition toward a low-range position, as denoted by position line “L”.Upon completion of speed synchronization, the first set of spline teethon range sleeve 88 move into meshed engagement with the external clutchteeth on second clutch plate 92 while the second set of spline teeth onrange sleeve 88 are maintained in engagement with the external splineteeth on drive plate 96. Thus, with range sleeve 88 located in itslow-range position, rear output shaft 32 is coupled for rotation withcarrier 86 and the low-range drive mode connection is establishedbetween input shaft 46 and rear output shaft 32.

To provide means for moving range sleeve 88 between its three distinctrange position, range shift mechanism 72 further includes a range fork102 coupled to range sleeve 88. Range actuator 48 is operable to moverange fork 102 for causing corresponding axial movement of range sleeve88 between its three range positions. Range actuator 48 is preferably anelectric motor arranged to move range sleeve 88 to a specific rangeposition in response to a control signal from controller 58 that isbased on the mode signal delivered to controller 58 from mode selectmechanism 56.

It will be appreciated that the synchronized range shift mechanismpermits “on-the-move” range shifts without the need to stop the vehiclewhich is considered to be a desirable feature. However, othersynchronized and non-synchronized versions of range clutch 44 can beused in substitution for the particular arrangement shown. Also, it iscontemplated that range clutch 44 and range actuator 48 can be removedentirely from transfer case 22 such that input shaft 46 would directlydrive rear output shaft 32 to define a one-speed version of theon-demand transfer case embodying the present invention.

Referring now primarily to FIGS. 2 and 3, transfer clutch 50 is shownarranged in association with front output shaft 42 in such a way that itfunctions to deliver drive torque from a transfer assembly 110 driven byrear output shaft 32 to front output shaft 42 for establishing thefour-wheel drive modes. Transfer assembly 110 includes a first sprocket112 fixed for rotation with rear output shaft 32, a second sprocket 114rotatably supported by bearings 116 on front output shaft 42, and apower chain 118 encircling sprockets 112 and 114. As will be detailed,transfer clutch 50 is a multi-plate clutch assembly 124 and modeactuator 52 is a magnetorheological clutch actuator 120 which togetherdefine a torque transfer mechanism.

Multi-plate clutch assembly 124 is shown to include an annular drum 126fixed for rotation with second sprocket 114, a hub 128 fixed via asplined connection 130 for rotation with front output shaft 42, and amulti-plate clutch pack 132 operably disposed between drum 126 and hub128. In particular, drum 126 has a first smaller diameter cylindricalrim 126A that is fixed (i.e., welded, splined, etc.) to sprocket 114 anda second larger diameter cylindrical rim 126B that is interconnected torim 126A by a radial plate segment 126C. Hub 128 is shown to include afirst smaller diameter hub segment 128A and a second larger diameter hubsegment 128B that are interconnected by a radial plate segment 128C.Clutch pack 132 includes a set of outer friction plates 134 that aresplined to outer rim 126B of drum 126 and which are alternativelyinterleaved with a set of inner friction plates 136 that are splined tohub segment 128B of clutch hub 128. Clutch assembly 124 further includesa first pressure plate 138 having a plurality ofcircumferentially-spaced and radially-extending tangs 140 that aredisposed in longitudinally-extending slots formed in hub segment 128Bprior to installation of clutch pack 132 such that a front face surface142 of each tang 140 abuts an end surface 144 of the slots so as todefine a fully retracted position of first pressure plate 138 relativeto clutch pack 132. Thus, first pressure plate 138 is coupled for commonrotation with clutch hub 128 and front output shaft 42. A secondpressure plate 146 is fixed via a splined connection 147 to rim 126B ofdrum 126 for rotation therewith. As seen, a plurality ofcircumferentially-spaced return springs 148 act between pressure plates138 and 146.

With continued reference to FIGS. 2 and 3, magnetorheological clutchactuator 120 is shown to generally include a thrust cam operator 150 andan electromagnetic energy source such as, for example, electromagneticcoil 152. Thrust cam operator includes a drive ring 154 fixed via aspline connection 156 for rotation with drum 126 and a reaction ring 158fixed to housing 60. Recesses 154A and 158A are formed in correspondingportions of drive ring 154 and reaction ring 158 and together define anannular chamber 160. Chamber 160 is filled with magnetorheological (MR)fluid 161, preferably of a high viscosity and of a type manufactured byLord Corporation, Erie, Pa. Drive ring 154 is supported on front outputshaft 42 via a bearing assembly 162 and has a front face surface 164 inengagement with second pressure plate 146. Seal rings 166 provide afluid tight seal between chamber 160 and housing 60. As seen, a firstcam disk 166 is secured within recess 154A such that it and drive ring154 together define a first thrust cam 167. Likewise, a second cam disk168 is secured within recess 158A such that it and reaction ring 158together define a second thrust cam 169. First cam disk 166 has afaceted face surface 166A communicating with MR fluid 161 in chamber160. Similarly, second cam disk 168 has a faceted face surface 168Acommunicating with MR fluid 161 in chamber 160. The faceted facesurfaces 166A, 168A are configured to include multiple angular and/orramped portions of different sizes and shapes which act as cam surfaces.

Electromagnetic coil 152 is secured to housing and is adapted to receiveelectric control signals from controller 58 for generating a magneticfield. In the absence of a magnetic field, first thrust cam 167 rotatesrelative to second thrust cam 169 in chamber 160. However, when MR fluid161 is exposed to a magnetic field upon activation of electromagneticcoil 152, its magnetic particles align with the field and increase theviscosity and, therefore, the shear strength of MR fluid 161. Thisincreased shear strength causes the fluid to exert an axial separationforce normal to the cam surfaces in the direction of relative rotation.Since second thrust cam 169 is axially restrained, the axial separationforce causes first thrust cam 167 to move axially for forcibly urgingsecond pressure plate 146 to move, in opposition to the biasing ofsprings 148, into engagement with clutch pack 132. Such engagement ofclutch pack acts to transfer drive torque from rear output shaft 32 tofront output shaft 42 through transfer mechanism 110. The magnitude ofthe axial separation force is proportional to the cam angles andgeometry of the opposing cam surfaces and the viscosity of MR fluid 161.

The biasing force of springs 148 limits axial movement of first thrustcam 167 as a function of the viscosity of MR fluid 161. For example, inits least viscous form, MR fluid 161 has no effect and first thrust cam167 rotates relative to second thrust cam 169 within chamber 160. In itsmost viscous form, MR fluid 161 has a large shear strength for inducingsufficient axial movement of first thrust cam 167 to fully engage clutchpack 132. However, axial movement of first thrust cam 167 is limited atfull engagement of clutch pack 132 and once having achieved that limit,the thrust cams function only to circulate the now highly viscous MRfluid 161 within chamber 160. Degrees of viscosity are achievablebetween the least viscous and most viscous form of MR fluid 161 and varywith the intensity of the magnetic field and, thus, with the magnitudeof the electric control signal sent to coil 152. As such, the value ofthe clutch engagement force induced by operator 150 and applied toclutch pack 132 of clutch assembly 124 can be adaptively varied as afunction of the magnitude of the electric control signal sent to coil152 between a no torque transfer condition (two-wheel drive mode with100% of drive torque delivered to rear output shaft 32) and atorque-split condition (part-time four-wheel drive mode with 50% ofdrive torque to front output shaft 42 and 50% to rear output shaft 32).Upon decease of the magnetic field strength, first thrust cam 167 isaxial biased by springs 148 against second pressure plate 146, therebyrelieving engagement of clutch pack 132 and biasing first thrust cam 167for movement to its released position.

In operation, when mode selector 56 indicates selection of the two-wheelhigh-range drive mode, range actuator 48 is signaled to move rangesleeve 88 to its high-range position and transfer clutch 50 ismaintained in a released condition with no electric signal sent to coil150 of magnetorheological clutch actuator 120, whereby all drive torqueis delivered to rear output shaft 32. If mode selector 56 thereafterindicates selection of a part-time four-wheel high-range mode, rangesleeve 88 is maintained in its high-range position and a predeterminedmaximum electrical control signal is sent by controller 58 to coil 152of magnetorheological clutch actuator 120 which causes axial movement offirst thrust cam 167 due to the resultant change in viscosity of MRfluid 161. Such action causes second pressure plate 146 to engage clutchpack 132 until a maximum clutch engagement force is exerted on clutchpack 132 for effectively coupling hub 128 to drum 126. In response tosuch movement of second pressure plate 146, return springs 148 arecompressed and act to forcibly locate first pressure plate 138 in itsfully retracted position where it acts as a reaction plate against whichclutch pack 132 is compressed.

If a part-time four-wheel low-range drive mode is selected, theoperation of transfer clutch 50 and magnetorheological clutch actuator120 are identical to that described above for the part-time high-rangedrive mode. However, in this mode, range actuator 48 is signaled tolocate range sleeve 88 in its low-range position to establish thelow-range drive connection between input shaft 46 and rear output shaft32.

When the mode signal indicates selection of the on-demand four-wheelhigh-range drive mode, range actuator 48 moves or maintains range sleeve88 in its high-range position and magnetorheological clutch actuator 120is placed in a ready or “stand-by” condition. In particular, the amountof drive torque sent to front output shaft 42 through transfer clutch 50with clutch actuator in its stand-by condition can be zero or a slightamount (i.e., in the range of 2–10%) as required for the specificvehicular application. This minimum stand-by torque transfer isgenerated by controller 58 sending a control signal to coil 152 having apredetermined minimum value. Thereafter, controller 58 determines whenand how much drive torque needs to be transferred to front output shaft42 based on tractive conditions and/or vehicle operatingcharacteristics, as detected by vehicle sensors 54. For example, FIG. 2illustrates a first speed sensor 180 which sends a signal to controller58 indicative of the rotary speed of rear output shaft 32 while a secondspeed sensor 182 sends a signal indicative of the rotary speed of frontoutput shaft 42. Controller 58 can vary the value of the electriccontrol signal sent to coil 152 between the predetermined minimum valueand the predetermined maximum value based on defined relationships ordetected characteristics such as, for example, the speed difference(ΔRPM) between shafts 32 and 42, vehicle acceleration, a brakingcondition and the steering angle.

Referring now to FIG. 4, an alternative version of a torque transfermechanism for use in transfer case 22 is shown. In particular, thistorque transfer mechanism includes multi-plate clutch assembly 124 whichis now associated with a different magnetorheological clutch actuator190. For purposes of clarity and brevity, similar components areidentified with common reference numerals throughout the drawings.Magnetorheological clutch actuator 190 is shown to generally include abrake operator 192 and an electromagnetic coil 152. Brake operator 192includes a first thrust cam 194 fixed for rotation with second pressureplate 146 and having a surface defining a series of first cams 196 thatare engaged or interdigitated with a series of second cams 198 formed ona surface of a second thrust cam 200. Second thrust cam 200 has anannular brake rotor 202 extending coaxially from its disk segment 204and which is disposed for rotation in an annular cavity 206 formed inhousing 60. A bearing assembly 208 rotatably supports rotor 202 fromhousing 60. Rotor 202 is formed with a circumferential recess in itsouter surface that defines a fluid chamber 210 with an inner wallsurface of housing 60 within cavity 206. A pair of laterally-spaced sealrings 212 are provided to seal chamber 210 which is filled with MR fluid161. Coil 152 is secured in housing 60 and is located in proximity tochamber 210. As previously detailed, activation of coil 152 results in amagnetic field being established in MR fluid 151.

The profile of cams 196 and 198 are such that second thrust cam 200rotates with first thrust cam 194 when MR fluid 161 has a low viscosity.Once an operating condition is detected that warrants actuation of thetorque transfer mechanism, controller 58 sends an appropriate controlsignal to coil 152. This results in an increase in the shear strength ofMR fluid 161 acting between housing 60 and rotor 202 of second thrustcam 200 which, in turn, exerts a reaction or brake torque on rotor 202.This braking of rotor 202 relative to housing 60 causes second thrustcam 200 to rotate relative to first thrust cam 194. As such, an axialseparation force is applied to first thrust cam 194 which isproportional to the cam angle between cams 196 and 198 and the magnitudeof the reaction torque exerted on rotor 202. This axial force then actsto cause second pressure plate 146 to exert a corresponding clutchengagement force on clutch pack 132, in opposition to the biasing forceexerted thereon by return springs 148.

As previously disclosed, controller 58 cam vary the value of theelectric control signal sent to electromagnetic coil 152 betweenpredetermined minimum and maximum values based on defined relationships(ΔRPM), vehicle operating characteristics and/or the mode signal frommode selector 56 so as to establish any one of the available drivemodes.

While both of the torque transfer mechanisms have been shown arranged onfront output shaft 42, it is evident that they could easily be installedon rear output shaft 32 for selectively transferring drive torque to atransfer assembly coupled to drive front output shaft 42. Furthermore,the present invention can be used as a torque transfer coupling in anall-wheel drive (AWD) vehicle to selectively and/or automaticallytransfer drive torque on-demand from the primary (i.e., front) drivelineto the secondary (i.e., rear) driveline. Likewise, in full-time transfercases equipped with an interaxle differential, transfer clutch 50 couldbe used to limit slip and bias torque across the differential.

Referring now to FIG. 5, a torque transfer mechanism, hereinafterreferred to as torque coupling 220, is shown to include a multi-plateclutch assembly 222 operably installed between an input member 224 andan output member 226, and a magnetorheological clutch actuator 228.Clutch assembly 222 includes a set of inner clutch plates 230 fixed viaa spline connection 232 for rotation with input member 224, a clutchdrum 234 fixed to output member 226, and a set of outer clutch plates236 fixed via a spline connection 238 to clutch drum 234. As seen, outerclutch plates 236 are alternatively interleaved with inner clutch plates230 to define a clutch pack. Drum 234 has a radial plate segment 240which functions as a reaction plate against which the interleaved clutchplates can be frictionally engaged. A bearing assembly 242 is shownsupporting drum 234 for rotation relative to input member 224.

Clutch actuator 228 is similar to clutch actuator 120 shown in FIG. 3and includes a thrust cam operator 150 and electromagnetic coil 152.However, in this arrangement, drive ring 154 is fixed via a splinedconnection 244 to input member 224. Again, common reference numerals areused identified components of clutch actuator 228 that are similar tocorresponding components of clutch actuator 120.

With reference to FIG. 6, an alternative torque transfer mechanism,hereinafter referred to as torque coupling 250, is shown to include amulti-plate clutch assembly 252 operably installed between a firstrotary member 254 and a second rotary member 256. Clutch assembly 252includes a clutch drum 258 fixed for rotation with first rotary member254, a hub 260 associated with second rotary member 256 and a clutchpack 262. Clutch pack 262 includes inner clutch plates 264 splined tohub 260 and which are interleaved with outer clutch plates 266 splinedto drum 258. Torque coupling 250 also includes a magnetorheologicalclutch actuator 270 that is similar to clutch actuator 190 shown in FIG.4. Clutch actuator 270 includes brake operator 192 and electromagneticcoil 152. Operation of MR clutch actuator 270 is substantially similarto that of clutch actuator 190 in that activation of coil 152 brakesrotation of rotor 202 for causing axial movement of first thrust cam 194for exerting a clutch engagement force on clutch pack 262. It iscontemplated that torque couplings 220 and 250 could be readily used invarious driveline applications including, without limitation, as theon-demand transfer clutch or the full-time bias clutch in 4WD transferunits, as an in-line coupling or power take-off unit, or as a limitedslip coupling in drive axles and AWD systems.

To illustrate an alternative power transmission device to which thepresent invention is applicable, FIG. 7 schematically depicts afront-wheel based four-wheel drivetrain layout 10′ for a motor vehicle.In particular, engine 18 drives a multi-speed transmission 20′ having anintegrated front differential unit 38′ for driving front wheels 34 viaaxle shafts 33. A transfer unit 35 is also driven by transmission 20′for delivering drive torque to the input member of an in-line torquetransfer coupling 300 via a drive shaft 30′. In particular, the inputmember of transfer coupling 300 is coupled to drive shaft 30′ while itsoutput member is coupled to a drive component of rear differential 28.Accordingly, when sensors indicate the occurrence of a front wheel slipcondition, controller 58 adaptively controls actuation of torquecoupling 300 such that drive torque is delivered “on-demand” to rearwheels 24. It is contemplated that torque transfer coupling 300 wouldinclude a multi-plate transfer clutch and a magnetorheological clutchactuator that are generally similar in structure and function to that ofany of the devices previously described herein. While shown inassociation with rear differential 28, it is contemplated that torquecoupling 300 could also be operably located for transferring drivetorque from transfer unit 35 to drive shaft 30′.

Referring now to FIG. 8, torque coupling 300 is schematicallyillustrated in association with an on-demand four-wheel drive systembased on a front-wheel drive vehicle similar to that shown in FIG. 7. Inparticular, an output shaft 302 of transaxle 20′ is shown to drive anoutput gear 304 which, in turn, drives an input gear 306 fixed to acarrier 308 associated with front differential unit 38′. To providedrive torque to front wheels 34, front differential unit 38′ includes apair of side gears 310 that are connected to front wheels 34 viaaxleshafts 33. Differential unit 38′ also includes pinions 312 that arerotatably supported on pinion shafts fixed to carrier 308 and which aremeshed with side gears 310. A transfer shaft 314 is provided to transferdrive torque from carrier 308 to a clutch hub 316 associated with amulti-pate clutch assembly 318. Clutch assembly 318 further includes adrum 320 and a clutch pack 322 having interleaved clutch plates operablyconnected between hub 316 and drum 320.

Transfer unit 35 is a right-angled drive mechanism including a ring gear324 fixed for rotation with drum 320 of clutch assembly 318 which ismeshed with a pinion gear 326 fixed for rotation with drive shaft 30′.As seen, a magnetorheological clutch actuator 328 is schematicallyillustrated for controlling actuation of clutch assembly 318. Accordingto the present invention, magnetorheological actuator 328 can be similarto any one of the various magnetorheological clutch actuators previouslydescribed in that an electromagnetic coil is supplied with electriccurrent for changing the viscosity of a magnetorheological fluid which,in turn, functions to control translational movement of a thrust cam forengaging clutch pack 322. In operation, drive torque is transferred fromthe primary (i.e., front) driveline to the secondary (i.e., rear)driveline in accordance with the particular mode selected by the vehicleoperator via mode selector 56. For example, if the on-demand 4WD mode isselected, controller 58 modulates actuation of magnetorheological clutchactuator 328 in response to the vehicle operating conditions detected bysensors 54 by varying the value of the electric control signal sent tothe electromagnetic coil. In this manner, the level of clutch engagementand the amount of drive torque that is transferred through clutch pack322 to the rear driveline through transfer unit 35 and drive shaft 30′is adaptively controlled. Selection of a locked or part-time 4WD moderesults in full engagement of clutch assembly 318 for rigidly couplingthe front driveline to the rear driveline. In some applications, themode selector 56 may be eliminated such that only the on-demand 4WD modeis available so as to continuously provide adaptive traction controlwithout input from the vehicle operator.

FIG. 9 illustrates a modified version of FIG. 8 wherein an on-demandfour-wheel drive system is shown based on a rear-wheel drive motorvehicle that is arranged to normally deliver drive torque to rear wheels24 while selectively transmitting drive torque to front wheels 34through a torque coupling 300A. In this arrangement, drive torque istransmitted directly from transmission output shaft 302 to transfer unit35 via a drive shaft 330 interconnecting input gear 306 to ring gear324. To provide drive torque to front wheels 34, torque coupling 300A isnow shown operably disposed between drive shaft 330 and transfer shaft314. In particular, clutch assembly 318 is arranged such that drum 320is driven with ring gear 324 by drive shaft 330. As such, actuation ofmagnetorheological clutch actuator 328 functions to transfer torque fromdrum 320 through clutch pack 322 to hub 316 which, in turn, drivescarrier 308 of front differential unit 38′ via transfer shaft 314.Again, the vehicle could be equipped with mode selector 56 to permitselection by the vehicle operator of either the adaptively controlledon-demand 4WD mode or the locked part-time 4WD mode. In vehicles withoutmode selector 56, the on-demand 4WD mode is the only mode available andwhich provides continuous adaptive traction control with input from thevehicle operator.

In addition to the on-demand 4WD systems shown previously, the powertransmission (magnetorheological clutch actuator and clutch assembly)technology of the present invention can likewise be used in full-time4WD systems to adaptively bias the torque distribution transmitted by acenter or “interaxle” differential unit to the front and reardrivelines. For example, FIG. 10 schematically illustrates a full-timefour-wheel drive system which is generally similar to the on-demandfour-wheel drive system shown in FIG. 9 with the exception that aninteraxle differential unit 340 is now operably installed betweencarrier 308 of front differential unit 38′ and transfer shaft 314. Inparticular, output gear 306 is fixed for rotation with a carrier 342 ofinteraxle differential 340 from which pinion gears 344 are rotatablysupported. A first side gear 346 is meshed with pinion gears 344 and isfixed for rotation with drive shaft 330 so as to be drivinglyinterconnected to the rear driveline through transfer unit 35. Likewise,a second side gear 348 is meshed with pinion gears 344 and is fixed forrotation with carrier 308 of front differential unit 38′ so as to bedrivingly interconnected to the front driveline. Torque transfermechanism 300B is shown operably installed between side gears 346 and348. In operation, when sensor 54 detects a vehicle operating condition,such as excessive interaxle slip, controller 58 adaptively controlsactivation of the electromagnetic coil associated withmagnetorheological clutch actuator 328 for controlling engagement ofclutch assembly 318, thereby adaptively controlling the torque biasingbetween the front and rear drivelines.

Referring now to FIG. 11, a full-time 4WD system is shown to include atransfer case 22′ equipped with an interaxle differential 350 between aninput shaft 46′ and output shafts 32′ and 42′. Differential 350 includesan input defined as a planet carrier 352, a first output defined as afirst sun gear 354, a second output defined as a second sun gear 356,and a gearset for permitting speed differentiation between first andsecond sun gears 354 and 356. The gearset includes meshed pairs of firstplanet gears 358 and second pinions 360 which are rotatably supported bycarrier 352. First planet gears 358 are shown to mesh with first sungear 354 while second planet gears 350 are meshed with second sun gear356. First sun gear 354 is fixed for rotation with rear output shaft 32′so as to transmit drive torque to rear driveline 12. To transmit drivetorque to front driveline 14, second sun gear 356 is coupled to atransfer assembly 110′ which includes a first sprocket 112′ rotatablysupported on rear output shaft 32′, a second sprocket 114′ fixed tofront output shaft 42′, and a power chain 118′.

Transfer case 22′ further includes a biasing clutch 50′ having amulti-plate clutch assembly 124′ and a mode actuator 52′ having amagnetorheological clutch actuator 120′. Clutch assembly 124′ includes adrum 126′ fixed for rotation with first sprocket 112′, a hub 128′ fixedfor rotation with rear output shaft 32′, and a multi-plate clutch pack132′ operably disposed therebetween. Magnetorheological clutch actuator120′ includes an electromagnetic coil that can be energized forcontrolling the viscosity of the magnetorheological fluid forcontrolling movement of a screw cam relative to clutch pack 132′.

A number of preferred embodiments have been disclosed to provide thoseskilled in the art an understanding of the best mode currentlycontemplated for the operation and construction of the presentinvention. The invention being thus described, it will be obvious thatvarious modifications can be made without departing from the true spiritand scope of the invention, and all such modifications as would beconsidered by those skilled in the art are intended to be includedwithin the scope of the following claims.

1. A torque transfer mechanism for use in a motor vehicle to transferdrive torque from a first rotary member to a second rotary member,comprising: a friction clutch operably interconnected between the firstand second rotary members; a magnetorheological clutch actuator forapplying a clutch engagement force to said friction clutch, said clutchactuator including a first cam driven by the first rotary member andhaving a first cam surface, a second cam having a second cam surfaceengaging said first cam surface and a rotor disposed in a chamber filledwith a magnetorheological fluid, and an electromagnetic energy sourcecapable of varying the viscosity of said magnetorheological fluid insaid chamber for retarding rotation of said second cam relative to saidfirst cam in response to electric control signals, said electromagneticenergy source is an electromagnetic coil secured to a non-rotary housingsuch that said chamber is defined between said rotor and said housing;and a control system for detecting an operational characteristic of themotor vehicle and generating said electric control signals in responsethereto.
 2. The torque transfer mechanism of claim 1 wherein activationof said electromagnetic energy source functions to brake rotation ofsaid second cam relative to said first cam such that said engagementbetween said first and second cam surfaces results in axial movement ofsaid first cam for applying said clutch engagement force on saidfriction clutch.
 3. The torque transfer mechanism of claim 1 furthercomprising a biasing mechanism for normally locating said first cam in areleased position when said electromagnetic energy source isdeactivated, and wherein activation of said electromagnetic energysource causes a resultant change in the viscosity of saidmagnetorheological fluid which causes an axial separation force betweensaid first and second cams to axially move said first cam toward anengaged position in opposition to the biasing of said biasing mechanism.4. The torque transfer mechanism of claim 1 wherein the first rotarymember is a driven shaft of a transfer case and the second rotary memberis an output shaft coupled to a driveline, said friction clutch operableto transfer drive torque from said driven shaft to said output shaft forestablishing a four-wheel drive mode in response to activation of saidelectromagnetic energy source.
 5. The torque transfer mechanism of claim1 wherein the first rotary member is a first output shaft of a transfercase that is coupled to a first driveline and the second rotary memberis a second output shaft that is coupled to a second driveline, whereinsaid transfer case includes an interaxle differential operably disposedbetween said first and second output shafts, and wherein said frictionclutch is operably disposed between said first and second output shafts.6. A power transmission device comprising: a rotary input member adaptedto receive drive torque from a power source; a first rotary outputmember adapted to provide drive torque to a first output device; asecond rotary output member adapted to provide drive torque to a secondoutput device; a gearset operably interconnecting said input member tosaid first and second output members; a torque transfer mechanismoperable for limiting speed differentiation between said first andsecond output members, said torque transfer mechanism including afriction clutch assembly operably disposed between said first outputmember and said second output member and a magnetorheological clutchactuator operable for applying a clutch engagement force to saidfriction clutch assembly, said magnetorheological clutch actuatorincluding a first cam driven by said first output member and having afirst cam surface, a second cam having a second cam surface engagingsaid first cam surface and a rotor disposed in a chamber filled with amagnetorheological fluid, and an electromagnet secured to a non-rotaryhousing such that said chamber is defined between said rotor and saidhousing, said electromagnet is capable of varying the viscosity of saidmagnetorheological fluid in said chamber for retarding rotation of saidsecond cam relative to said first cam in response to electric controlsignals; and a control system for detecting an operationalcharacteristic of a motor vehicle and generating said electric controlsignals in response thereto.
 7. The power transmission device of claim 6wherein said friction clutch assembly includes an interleaved clutchpack having a first set of clutch plates fixed for rotation with saidfirst output member and a second set of clutch plates fixed for rotationwith said second output member, and a pressure plate, and wherein axialmovement of said first cam causes said pressure plate to apply saidclutch engagement force on said clutch pack.
 8. The power transmissionof claim 6 wherein said input member is an input shaft of a transfercase, said first output member is a first output shaft of said transfercase, and said second output member is a second output shaft of saidtransfer case, and wherein said gearset is an interaxle differentialoperably interconnecting said input shaft to said first and secondoutput shafts.
 9. The power transmission of claim 6 wherein said controlsystem establishes the value of said electric control signal based on arotary speed difference between first and second output members, andwherein said control signal is operable to vary the viscosity of saidmagnetorheological fluid in said chamber for causing relative rotationbetween said first and second cams which results in axial movement ofsaid first cam relative to said friction clutch assembly.
 10. A transfercase for use in a motor vehicle having a powertrain and first and seconddrivelines, comprising: an input shaft driven by the powertrain; a firstoutput shaft adapted for connection to the first driveline; a secondoutput shaft adapted for connection to the second driveline; aninteraxle differential operably interconnecting said input shaft to saidfirst and second output shafts; a torque transfer mechanism operable forlimiting speed differentiation between said first and second outputshafts, said torque transfer mechanism including a first member coupledto said first output shaft, a second member coupled to second outputshaft, a friction clutch assembly operably disposed between said firstmember and said second member, and a clutch actuator operable forapplying a clutch engagement force on said friction clutch assembly,said clutch actuator including a first cam driven by said first outputshaft and having a first cam surface, a second cam having a second camsurface engaging said first cam surface and a rotor disposed in achamber filled with a magnetorheological fluid, and an electromagneticenergy source capable of varying the viscosity of saidmagnetorheological fluid in said chamber for retarding rotation of saidsecond cam relative to said first cam in response to electric controlsignals, said electromagnetic energy source is an electromagnetic coilsecured to a non-rotary housing such that said chamber is definedbetween said rotor and said housing; and a control system for detectingan operational characteristic of the motor vehicle and generating saidelectric control signals in response thereto.
 11. The transfer case ofclaim 10 wherein said control system establishes the value of saidelectric control signal based on a rotary speed difference between saidfirst output shaft and said second output shaft, and wherein saidcontrol signal is operable to vary the viscosity of saidmagnetorheological fluid in said chamber for causing relative rotationbetween said first and second cams which results in axial movement ofsaid first cam relative to said friction clutch assembly.
 12. A clutchactuator for controlling the magnitude of a clutch engagement forceexerted on a clutch pack that is operably disposed between a firstrotary member and a second rotary member, comprising: a first cam fixedfor rotation with the first rotary member and having a first camsurface; a second cam having a rotor and a second cam surface reactingagainst said first cam surface; a magnetorheological fluid in a chamberformed between a non-rotary housing and said rotor; and anelectromagnetic coil secured to said housing, wherein saidelectromagnetic coil is selectively energized for varying the viscosityof said magnetorheological fluid to correspondingly vary a reactionforce between said first and second cams, thereby imparting relativeaxial movement of said first cam for initiating engagement of saidclutch pack.
 13. The clutch actuator of claim 12 wherein the viscosityof said magnetorheological fluid is manipulated to induce a brake forceon said rotor for imparting said reaction force between said first andsecond cams.
 14. A torque transfer mechanism for use in a motor vehicleto transfer drive torque from a first rotary member to a second rotarymember, comprising: a friction clutch operably interconnected betweenthe first and second rotary members; a magnetorheological clutchactuator for applying a clutch engagement force to said friction clutch,said clutch actuator including a first cam driven by the first rotarymember having a first cam surface, a second cam having a second camsurface engaging said first cam surface and a rotor disposed in achamber filled with a magnetorheological fluid, and an electromagneticenergy source capable of varying the viscosity of saidmagnetorheological fluid in said chamber for retarding rotation of saidsecond cam relative to said first cam in response to electric controlsignals, and a biasing mechanism for normally locating said first cam ina released position when said electromagnetic energy source isdeactivated, such that activation of said electromagnetic energy sourcecauses a resultant change in the viscosity of said magnetorheologicalfluid which causes an axial separation force between said first andsecond cams to axially move said first cam toward an engaged position inopposition to the biasing of said biasing mechanism; and a controlsystem for detecting an operational characteristic of the motor vehicleand generating said electric control signals in response thereto. 15.The torque transfer mechanism of claim 14 wherein activation of saidelectromagnetic energy source functions to brake rotation of said secondcam relative to said first cam such that said engagement between saidfirst and second cam surfaces results in axial movement of said firstcam for applying said clutch engagement force on said friction clutch.16. The torque transfer mechanism of claim 14 wherein the first rotarymember is a driven shaft of a transfer case and the second rotary memberis an output shaft coupled to a driveline, said friction clutch operableto transfer drive torque from said driven shaft to said output shaft forestablishing a four-wheel drive mode in response to activation of saidelectromagnetic energy source.
 17. The torque transfer mechanism ofclaim 14 wherein the first rotary member is a first output shaft of atransfer case that is coupled to a first driveline and the second rotarymember is a second output shaft that is coupled to a second driveline,wherein said transfer case includes an interaxle differential operablydisposed between said first and second output shafts, and wherein saidfriction clutch is operably disposed between said first and secondoutput shafts.
 18. A power transmission device comprising: a rotaryinput member adapted to receive drive torque from a power source; afirst rotary output member adapted to provide drive torque to a firstoutput device; a second rotary output member adapted to provide drivetorque to a second output device; a gearset operably interconnectingsaid input member to said first and second output members; a torquetransfer mechanism operable for limiting speed differentiation betweensaid first and second output members, said torque transfer mechanismincluding a friction clutch assembly operably disposed between saidfirst output member and said second output member and amagnetorheological clutch actuator operable for applying a clutchengagement force to said friction clutch assembly, saidmagnetorheological clutch actuator including a first cam driven by saidfirst output member and having a first cam surface, a second cam havinga second cam surface engaging said first cam surface and a rotordisposed in a chamber filled with a magnetorheological fluid, anelectromagnet capable of varying the viscosity of saidmagnetorheological fluid in said chamber for retarding rotation of saidsecond cam relative to said first cam in response to electric controlsignals, and a biasing mechanism for normally locating said first cam ina released position when said electromagnetic energy source isdeactivated, such that activation of said electromagnetic energy sourcecauses a resultant change in the viscosity of said magnetorheologicalfluid which causes an axial separation force between said first andsecond cams to axially move said first cam toward an engaged position inopposition to the biasing of said biasing mechanism; and a controlsystem for detecting an operational characteristic of a motor vehicleand generating said electric control signals in response thereto. 19.The power transmission of claim 18 wherein said input member is an inputshaft of a transfer case, said first output member is a first outputshaft of said transfer case, and said second output member is a secondoutput shaft of said transfer case, and wherein said gearset is aninteraxle differential operably interconnecting said input shaft to saidfirst and second output shafts.
 20. The power transmission of claim 18wherein said control system establishes the value of said electriccontrol signal based on a rotary speed difference between first andsecond output members, and wherein said control signal is operable tovary the viscosity of said magnetorheological fluid in said chamber forcausing relative rotation between said first and second cams whichresults in axial movement of said first cam relative to said frictionclutch assembly.
 21. A transfer case for use in a motor vehicle having apowertrain and first and second drivelines, comprising: an input shaftdriven by the powertrain; a first output shaft adapted for connection tothe first driveline; a second output shaft adapted for connection to thesecond driveline; an interaxle differential operably interconnectingsaid input shaft to said first and second output shafts; a torquetransfer mechanism operable for limiting speed differentiation betweensaid first and second output shafts, said torque transfer mechanismincluding a first member coupled to said first output shaft, a secondmember coupled to second output shaft, a friction clutch assemblyoperably disposed between said first member and said second member, anda clutch actuator operable for applying a clutch engagement force onsaid friction clutch assembly, said clutch actuator including a firstcam driven by said first output shaft and having a first cam surface, asecond cam having a second cam surface engaging said first cam surfaceand a rotor disposed in a chamber filled with a magnetorheologicalfluid, an electromagnetic energy source capable of varying the viscosityof said magnetorheological fluid in said chamber for retarding rotationof said second cam relative to said first cam in response to electriccontrol signals, and a biasing mechanism for normally locating saidfirst cam in a released position when said electromagnetic energy sourceis deactivated, and wherein activation of said electromagnetic energysource causes a resultant change in the viscosity of saidmagnetorheological fluid which causes an axial separation force betweensaid first and second cams to axially move said first cam toward anengaged position in opposition to the biasing of said biasing mechanism;and a control system for detecting an operational characteristic of themotor vehicle and generating said electric control signals in responsethereto.